Shell Key Preserve: an undeveloped barrier island off Tierra Verde, known for shelling and sandbars. Ferries and tours leave from Tierra Verde and John's Pass.
Egmont Key: a state park at the mouth of Tampa Bay with a lighthouse, old fort ruins and snorkeling off the beach. It's only reachable by boat.
Caladesi Island: a state park beach north of Clearwater, reached by boat or from Honeymoon Island.
There's little or no shade or food on the islands, so bring water, sunscreen and snacks. Check the booking page for the return time.